let binaryString = String(2, radix: 2)
let other = String(count: 8 - binaryString.characters.count, repeatedValue: Character("0")) + binaryString

String构造函数在转换之前**无法知道Integer有多少个0位，你必须自己处理。
顺便说一下，Int也有radix构造函数，用于将字符串转换为Int：

Int("0000010", radix: 2) // Returns 2

"雨燕5"

var binaryString = String(2, radix: 2)
binaryString = String(repeating: "0", count: 8 - binaryString.count) + binaryString

FixedWidthInteger协议的文档提供了一个表示数字二进制的有趣示例：

extension FixedWidthInteger {
    var binaryString: String {
        var result: [String] = []
        for i in 0..<(Self.bitWidth / 8) {
            let byte = UInt8(truncatingIfNeeded: self >> (i * 8))
            let byteString = String(byte, radix: 2)
            let padding = String(repeating: "0",
                                 count: 8 - byteString.count)
            result.append(padding + byteString)
        }
        return "0b" + result.reversed().joined(separator: "_")
    }
}

print(Int16.max.binaryString) // Prints "0b01111111_11111111"
print((101 as UInt8).binaryString) // Prints "0b11001001"